A variety of user-recordable optical discs is now available, including DVD-RAM, CD-RW, and mini discs. Recordable optical discs such as these have grooves formed along plural spiral or concentric tracks with a groove surface made from phase change material or magneto-optical material. Addresses identifying specific locations on the disc are also pre-recorded to the tracks using unrewritable marks. As recording density of the disc increases, so does the need for additional address information, and it is therefore essential to provide the address information as efficiently as possible.
Japanese Patent Laid-Open Publication 8-315426 teaches one way to accomplish this by forming patterns corresponding to address signals in non-contiguous areas in the grooves. A “pattern corresponding to an address signal” refers to a binary signal that inverts in each non-contiguous area. These non-contiguous areas are used as signals indicating if information is present or not (yes or no), and therefore cannot be used to efficiently provide address information.
A different addressing method as disclosed in WPO laid-open document WO 01/52250 as a second example of the prior art teaches a technology for providing a wave-like wobble groove on the optical disc and selectively varying the wobble pattern. By encoding two or more meanings, that is, different information, using these wobble variations, address information can be provided more efficiently.
These wobble pattern variations are achieved in the second example by suddenly changing troughs (or peaks) of the groove to peaks (or troughs). The greater the difference in the slope between where there is a sudden change and where there is a gradual change in wobble pattern, the larger information component and the easier detection is during data playback. However, a laser polarizer with a relatively wide frequency band is required to manufacture optical discs having grooves with such sudden changes in the wobble pattern.